The present invention relates generally to communication systems, and more particularly to a system and method for reducing leading edge transients in optical signals by using at least one co-propagating pump signal in an optical amplifier receiving the optical signal.
To reduce noise transfer from a pump signal to an optical signal being amplifier, amplifiers used in optical communication systems typically attempt to minimize the amount of time a pump signal interacts with the optical signal traversing the gain medium of the amplifier. As a result, most optical amplifiers implement only pump signals that propagate counter to the direction of propagation of the optical signal. Communication systems implementing exclusively counter-propagating pump signals tend to create a leading edge transient within the optical signal. This leading edge transient typically results in a power spike at the leading edge of the optical signal. Once created, this power spike continues to increase in magnitude as the optical signal passes through additional amplifiers and/or amplification stages. The magnitude of these power spikes can be sufficiently large to cause transmission errors and in some cases cause damage to the receivers of the optical communication system.
The present invention recognizes a need for a system and method for reducing leading edge transients in optical signals using at least one co-propagating pump signal in an optical amplifier. In accordance with the present invention, an apparatus and method for reducing leading edge transients in optical signals is provided that reduce or eliminate at least some of the shortcomings associated with prior approaches.
In one embodiment, an optical amplifier comprises at least one amplification stage having a saturation recovery time of less than one millisecond. The amplification stage comprises a gain medium operable to receive at least one pump signal and to receive from a multiple span communication link an optical signal comprising a leading edge. The at least one pump signal and the optical signal travel in the same direction at approximately the same speed through at least a portion of the gain medium. In one particular embodiment, the leading edge of the optical signal after passing through a plurality of amplifiers when received by a receiver coupled to the communication link comprises a peak power that is no more than ten (10) times the average power of the optical signal at the receiver.
In another embodiment, an optical amplifier operable to be coupled to a multiple span communication link, the amplifier comprises a gain medium operable to receive at least one pump signal and an optical signal comprising a leading edge. The at least one pump signal and the optical signal interact while traveling through at least a portion of the gain medium in the same direction. In one particular embodiment, the leading edge of the optical signal when received by a receiver coupled to an end of the communication link comprises a peak power that is no more than ten times the average power of the optical signal received. This condition holds even where a saturation recovery time of the amplifier is less than a time period between the leading edge of the optical signal received by the amplifier and a trailing edge of an optical signal received just prior to the optical signal.
In yet another embodiment, an optical amplifier comprises at least one amplification stage. The amplification stage comprises a gain medium comprising at least length of two hundred (200) meters. The amplification stage is operable to receive at least one pump signal and to receive from a multiple span communication link an optical signal comprising a leading edge. The at least one pump signal and the optical signal travel in the same direction at approximately the same speed through at least a portion of the gain medium. In one particular embodiment, the amplification stage comprises a Raman amplification stage. In that embodiment, the at least one pump signal goes from a non-saturated operating state to an at least partially saturated operating state while the leading edge traverses the amplification stage to reduce an overshoot in gain imparted to the leading edge.
In still another embodiment, an optical amplifier comprises at least one amplification stage. The amplification stage comprises a gain medium operable to receive at least one pump signal and to receive from a multiple span communication link an optical signal comprising a leading edge. The at least one pump signal and the optical signal travel in the same direction at approximately the same speed through at least a portion of the gain medium. The amplification stage comprises a Raman amplification stage. In one particular embodiment, the leading edge of the optical signal substantially overlaps with a portion of the pump signal through at least a majority of the gain medium to reduce an overshoot in gain imparted to the leading edge by the at least one pump signal.
In a method embodiment, a method of amplifying an optical signal in a multiple span communication link comprises receiving an optical signal at a gain medium of the amplification stage having a saturation recovery time of less than one millisecond. The optical signal comprising a leading edge. The method also includes, introducing at least one pump signal to the gain medium to interact with the optical signal. The optical signal and at least a portion of the at least one pump signal travel in the same direction and at approximately the same speed through at least a portion of the gain medium. In one particular embodiment, the leading edge of the optical signal when received by a receiver coupled to the communication link after passing through a plurality of amplifiers comprises a peak power that is no more than ten (10) times the average power of the optical signal at the receiver.
In another embodiment, a method of amplifying an optical signal in a multiple span communication link comprises receiving at a gain medium of an optical amplification stage an optical signal comprising a leading edge. The method also comprises receiving at a gain medium of an optical amplification stage an optical signal comprising a leading edge. The method further comprises introducing at least one pump signal to the gain medium to interact with the optical signal. The optical signal and at least a portion of the at least one pump signal travel through at least a portion of the gain medium in the same direction. In one particular embodiment, the leading edge of the optical signal when received by a receiver coupled to an end of the communication link comprises a peak power that is no more than ten times the average power of the optical signal received. This condition holds even where a saturation recovery time of the amplifier is less than a time period between the leading edge of the optical signal received by the amplifier and a trailing edge of an optical signal received just prior to the optical signal.
In yet another method embodiment, a method of amplifying an optical signal in a multiple span communication link comprises receiving at a gain medium of an optical amplification stage an optical signal comprising a leading edge. The method also comprises introducing at least one pump signal to the gain medium to interact with the optical signal. The gain medium comprises a length of two hundred (200) meters. The optical signal and at least a portion of the at least one pump signal travel through at least a portion of the gain medium in the same direction. The amplification stage comprises a Raman amplification stage. In one particular embodiment, the at least one pump signal goes from a non-saturated operating state to an at least partially saturated operating state while the leading edge traverses the amplification stage to reduce an overshoot in gain imparted to the leading edge of the optical signal.
Depending on the specific features implemented, particular embodiments may exhibit some, none, or all of the following technical advantages. Various embodiments reduce leading edge transients in optical signals being communicated. This can help prevent the formation of transmission errors and protect receivers from being damaged by large transient power spikes. These techniques can be particularly useful, for example, during a signal turn-on period or where the amplifier sees bursty traffic patterns. Other embodiments enable the use of a co-propagating pump signal that travels at approximately the same speed as the optical signal, while maintaining an acceptable optical noise figure.
Unlike optical systems that rely exclusively on feedback control to deal with leading edge transients, various embodiments described herein substantially reduce the formation of even the initial power spike. Unlike optical systems reduce leading edge transients solely by regulating speed of the turn-up of signal power, various embodiments described herein allow for immediate signal power turn-up. Moreover, the embodiments described herein can accommodate bursty traffic patterns that can present problems for systems relying solely on regulation of signal turn-up power.
If desired, embodiments described herein can be combined with other transient signal control methods, such as using feedback control to regulate pump powers, or regulating signal turn on speeds.
Other technical advantages will be readily apparent to one skilled in the art from the following figures, descriptions and claims. Moreover, while specific advantages have been enumerated above, various embodiments may include all, some or none of the enumerated advantages.